White-Throated Sparrows

White-throated Sparrows are a polymorphic species which come in a white-stripe morph and a tan-stripe morph. Plumage is not sexually dimorphic and both males and females can have either the white or tan morph plumage. White-throated Sparrow mate disassortitavely by morph so that a white morph always pairs with a tan morph creating two pairing possibilities and four functional sexes. More information on White-throated Sparrows can be found here.

Figure 1.Presumed white morph (left) and tan morph (right) White-throated SparrowsFigure 1.Presumed white morph (left) and tan morph (right) White-throated Sparrows

Figure 1.Presumed white morph (left) and tan morph (right) White-throated Sparrows

Study Sites

The study sites where White-throated Sparrows were banded and data collected are Eastern Hills and Emerald Hill in Clarksville, TN. Both consist of small fragments of deciduous woodland imbedded within an urban landscape, suitable habitat for White-throated Sparrows.

Figure 2. A map including the study sites of Eastern Hills and Emerald Hill within Clarksville, TN

Sex Differences and Morph Differences

Even though plumage is not sexually dimorphic in White-throated Sparrows, there may be other differences by sex. Male White-throated Sparrows generally winter further north than female White-throated Sparrows. As such, there may be differences in wing morphology. Morph may also affect development, so differences could be seen between morphs.

Figure 3. Wing chord length broken down by sex (left) and both sex and morph (right).Figure 3. Wing chord length broken down by sex (left) and both sex and morph (right).

Figure 3. Wing chord length broken down by sex (left) and both sex and morph (right).

Analysis

A t-test would be used if sex were the only categorical variable. To test whether both sex and morph affect wing chord length, we will use a two-way ANOVA.

Table 1. Mean wing chord length (mm) and standard deviation for male and female White-throated Sparrows
PCRsex n mean sd
F 69 70.0 2
M 100 73.6 2
Table 2. Mean wing chord length (mm) and standard deviation for male and female white and tan morph White-throated Sparrows
PCRsex PCRMorph n mean sd
F TS 41 69.7 2
F WS 28 70.5 2
M TS 44 72.9 2
M WS 56 74.1 2

Wing Chord Length and Sex

Data looks to be normally distributed, so a Welch’s t-test was utilized. Mean wing chord length for female White-throated Sparrows (70.0mm) was significantly different (t = -11.5, df = 147.6, p < 0.001) from mean wing chord length for male White-throated Sparrows (73.57).

Figure 4. Histogram showing distribution of wing chord length for male and female White-throated Sparrows

Figure 4. Histogram showing distribution of wing chord length for male and female White-throated Sparrows 

## 
##  Welch Two Sample t-test
## 
## data:  Wing by PCRsex
## t = -11.515, df = 147.58, p-value < 2.2e-16
## alternative hypothesis: true difference in means between group F and group M is not equal to 0
## 95 percent confidence interval:
##  -4.182662 -2.957338
## sample estimates:
## mean in group F mean in group M 
##           70.00           73.57

Wing Chord Length, Sex, and Morph

Wing chord length differed significantly for both sex (df = 1, F = 28.3 ,p < 0.001) and morph (df = 1, F = 125, p < 0.001) in White-throated Sparrows. Males had a longer wing chord length than females, and white morphs had a longer wing chord length than tan morphs. There were no interactions.
Figure 5. Plots checking assumptions for wing chord length model

Figure 5. Plots checking assumptions for wing chord length model 

## Analysis of Variance Table
## 
## Response: Wing
##                  Df Sum Sq Mean Sq  F value    Pr(>F)    
## PCRMorph          1 104.85  104.85  28.3605 3.254e-07 ***
## PCRsex            1 462.58  462.58 125.1216 < 2.2e-16 ***
## PCRMorph:PCRsex   1   1.43    1.43   0.3865     0.535    
## Residuals       165 610.01    3.70                       
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Call:
## lm(formula = Wing ~ PCRMorph * PCRsex, data = feather_clean)
## 
## Residuals:
##     Min      1Q  Median      3Q     Max 
## -7.1071 -1.1071  0.1136  1.1136  4.5000 
## 
## Coefficients:
##                    Estimate Std. Error t value Pr(>|t|)    
## (Intercept)         69.6585     0.3003 231.975  < 2e-16 ***
## PCRMorphWS           0.8415     0.4714   1.785   0.0761 .  
## PCRsexM              3.2278     0.4174   7.734 9.78e-13 ***
## PCRMorphWS:PCRsexM   0.3793     0.6101   0.622   0.5350    
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 1.923 on 165 degrees of freedom
## Multiple R-squared:  0.4825, Adjusted R-squared:  0.4731 
## F-statistic: 51.29 on 3 and 165 DF,  p-value: < 2.2e-16

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